For obtaining desired characteristics, electrophotographic photosensitive materials having various constructions commensurate with the electrophotographic processes being employed.
Typical electrophotographic photosensitive materials include a photosensitive material comprising a support having formed thereon a photoconductive layer and a photosensitive material comprising a support having formed thereon a photosensitive layer and an insulating layer formed on the surface of the photosensitive layer. An electrophotographic photosensitive material comprising a support and at least one photoconductive layer is used for forming images by the most general electrophotographic process, that is, by electrostatic charging, image exposure, and development (and further, if necessary, image transferring).
Furthermore, a process of using an electrophotographic photosensitive material for directly producing a light-sensitive offset printing plate has been widely practiced.
A binder for forming the photoconductive layer of an electro-photographic photosensitive material is required to have various electrostatic characteristics. The binder should be excellent in film-forming property. The binder should adequately disperse therein photoconductive powders. The photoconductive layer formed using the binder should have good adhesive property with respect to the base material, should be excellent in static charging characteristic, show low dark decay, show large light decay, and show low pre-exposure fatigue, and further stably retain these properties even though humidity may vary at photographing. Moreover, the binder is required to have excellent photographic properties.
As conventionally known resins, there are silicone resins as described in Japanese Patent Publication No. 6670/59, styrene-butadiene resins as described in Japanese Patent Publication No. 1960/60, alkyd resins, maleic acid resins, polyamide resins as described in Japanese Patent Publication No. 11219/60, vinyl acetate resins as described in Japanese Patent Publication No. 2425/66, vinyl acetate copolymer resins as described in Japanese Patent Publication No. 2426/66, acryl resins as described in Japanese Patent Publication No. 11216/60, acrylic acid ester copolymers as described in Japanese Patent Publication Nos. 11219/60, 8510/61, and 13946/66, etc.
However, the electrophotographic photosensitive materials using these resins have the following problems. That is, (1) the affinity with photoconductive powders is deficient and the coating composition containing the resin is poor in dispersibility, (2) the photoconductive layer formed using the resin is low in static-charging property, (3) the image portion of the copy formed has poor in quality (in particular, dot reproducibility and resolving power), (4) the images formed are liable to be influenced by the environment (e.g., high-temperature and high-humidity condition or low-temperature and low-humidity condition) at the formation of copies, and (5) the film strength of the photosensitive layer and the adhesivity of the photosensitive layer are insufficient and hence if the photosensitive material is used, in particular, for producing an offset master, separation, etc., of the photosensitive layer occur to make it impossible to make a large number of prints.
Various methods for improving the electrostatic characteristics of photoconductive layers have been proposed. For example, Japanese Patent Publication Nos. 6878/67 and 3073/70 disclose a method of incorporating a compound having a carboxy group or nitro group on the aromatic ring or furan ring, or incorporating an anhydride of a dicarboxylic acid. However, even the electrophotographic photosensitive material improved by the aforesaid method remains insufficient in the electrostatic characteristics, and electrophotographic photosensitive materials having, in particular, excellent light decay characteristics have not yet been obtained even by the aforesaid method.
As one approach for improving the deficiency in sensitivity of the electrophotographic photosensitive material, a method of adding a large amount of sensitizing dye(s) to the photoconductive layer has hitherto been employed. However, the electrophotographic photosensitive materials prepared by employing the aforesaid method have problems. For example, the whiteness is greatly reduced to reduce the quality as recording materials and, as the case may be, the photosensitive materials cause deterioration of dark decay, whereby sufficient images are not obtained.
As another approach, a method involving controlling the average molecular weight of the binder resin used in the photoconductive layer is disclosed in Japanese Patent Application (OPI) No. 10254/85 (the term "OPI" as used herein indicates an "unexamined published Japanese patent application"). That is, there is described a technique of improving the electrophotographic characteristics (in particular, good repeated reproducibility as PPC photosensitive materials), humidity resistance, etc., by using an acryl resin having an acid value of from 4 to 50 and an average molecular weight of from 1.times.10.sup.3 to 1.times.10.sup.4 in combination with an acryl resin having an acid value of from 4 to 50 and an average molecular weight of from 1.times.10.sup.4 to 2.times.10.sup.5.
Furthermore, earnest investigations in photosensitive plates for making lithographic printing plates using electrophotographic photosensitive material have been made and as binder resins for photoconductive layers using both the electrostatic characteristics as electrophotographic photosensitive material and the printing characteristics as photosensitive plate for making the printing plate it is known that certain materials have the effect of improving the desensitization for photoconductive layers. That is, there are known, for example, a system of using a resin having a molecular weight of from 1.8.times.10.sup.4 to 1.times.10.sup.5 and a glass transition point (Tg) of from 10.degree. C. to 80.degree. C. prepared by copolymerizing a (meth)acrylate type monomer and other monomer in the presence of fumaric acid and a copolymer composed of a (meth)acrylate type monomer and a monomer other than fumaric acid as described in Japanese Patent Publication No. 31011/75, a system of using a ternary copolymer containing a (meth)acrylic acid ester having a substituent which has a carboxylic acid group at 7 atoms apart from the ester bond as described in Japanese Patent Application (OPI) No. 54027/78, a system of using a quaternary or pentamerous copolymer containing acrylic acid and hydroxyethyl (meth)acrylate as described in Japanese Patent Application (OPI) No. 202544/82, and a system of using a ternary copolymer containing a (meth)acrylic acid ester having an alkyl group of from 6 to 12 carbon atoms as a substituent and a vinyl monomer having a carboxylic acid as described in Japanese Patent Application (OPI) No. 68046/83.
However, even in the case of using the aforesaid resins which are said to have the effects of improving electrostatic characteristics, humidity resisting property, and durability, there are problems in electrostatic characteristics such as, in particular, electrostatic charging property, dark reduction retentivity, and photosensitivity and the smoothness of photoconductive layers and thus the use of these resins is yet insufficient for practical use.
Also, in the actual evaluation of binder resins which are developed for electrophotographic photosensitive plates for making lithographic printing plates, there are problems in the aforesaid electrostatic characteristics and background stains in prints.